x10.1 Locality Newton's Law of Universal Gravitation was always open to the complaint that it involved \Action at a Distance", contrary to the Principle of Locality. But it was very well established empirically, and had to be accepted. Similarly in contemporary quantum me- chanics we seem to have correlations between measurements that defy the Principle of Locality, but have to be accepted none the less.1 Although locality is a characteristic mark of causal con- nexion, it is not, as Hume supposed,2 an essential one. Nor is it merely a uniformity we have found to hold for the most part|else we should feel little compunction in accepting that in some cases it happened not to hold. It is clearly an a priori principle, though not an absolutely necessary one.